This invention relates to a turbofan engine.
Turbofan engines are also referred to as bypass engines or dual-flow turbine jet engines or fan engines. They are characterized in that the fan (also termed “blower”) is driven by a multi-stage low-pressure turbine arranged behind a core engine, where a major proportion of the air mass ingested by the engine is accelerated past the core engine. The engine forms here a primary flow duct through the core engine and a secondary flow duct for the outer airflow routed past the core engine.
A two-shaft turbofan engine is classically designed such that it has two shafts, i.e. a first shaft that couples the low-pressure turbine to the fan, and is also referred to as low-pressure shaft, and a second shaft that couples the high-pressure turbine to the high-pressure compressor, and is also referred to as high-pressure shaft. The shaft design is such that the low-pressure shaft is passed through the high-pressure shaft, i.e. they are installed coaxially.
The general intention must be to provide engines with a fan having a large diameter, in order to increase the bypass ratio and thereby make the engines more efficient and less noisy. There is therefore a need to develop engines with fans (fan stages) with increasingly larger dimensions.
An increase in the fan diameter however causes problems. On the one hand it must be taken into consideration that aircraft engines are frequently installed under the wings of an aircraft. It must be ensured here that a minimum clearance from the ground is provided, as illustrated in FIG. 5. The permitted size of the fan therefore depends, among other things, on the vertical position of the wing on the aircraft, on the vertical clearance of the engine from the wing and on the ground clearance to be considered.
On the other hand, the situation is such that the fan turns more slowly as the diameter increases. The low-pressure shaft turns correspondingly more slowly as the fan diameter increases. As a result, the load (torque) to be transmitted by the low-pressure shaft increases, for which reason the low-pressure shaft must be designed thicker. This in turn leads to an increased installation space for the low-pressure shaft. Since the low-pressure shaft is installed coaxially with the high-pressure shaft, this leads at the same time to an increased installation space in the core engine. This however runs counter to the general efforts being made in engine technology to keep reducing the size of the core engine by continually increasing the compression ratios.
A turbofan engine is known from US 2006/0185346 A1 that has two low-pressure turbines and two fans, each connected to one another by a low-pressure shaft, where the air mass exiting the core engine initially flows through the one low-pressure turbine and then through the other low-pressure turbine. The two low-pressure shafts are here arranged at an angle. This angled arrangement must be deemed disadvantageous, since it prevents straight thrust vectors from being achieved. Also, the aforementioned installation space problem in the core engine is not remedied, since one of the two low-pressure shafts runs coaxially to the high-pressure shaft of the core engine.
A turbofan engine is known from US 2010/0011741 A1 that has a low-pressure shaft coupled at its axial front end via a gearbox to two parallel-arranged shafts which each drive their own fan. With this engine too, the aforementioned installation space problem in the area of the core engine is not remedied, since the low-pressure shaft is passed through the core engine.